Field of the Invention
The present invention relates to an active matrix display and, more particularly, to an active matrix organic light emitting display utilizing a demultiplexer to supply data voltages from data drivers to pixels.
Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (hereinafter, abbreviated to “OLEDs”) capable of emitting light by itself, and has advantages, such as fast response time, high light emitting efficiency, high luminance, and wide viewing angle.
The OLED serving as a self-emitting element has a structure shown in FIG. 1. The OLED includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied between the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
The organic light emitting display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels depending on the grayscale of input video data. Each pixel includes a driving thin film transistor (TFT) to control a driving current flowing in the OLED depending on a gate-source voltage of the driving TFT, a capacitor to hold a gate potential of the driving TFT constant during one frame, and a switching TFT to store a data voltage in the capacitor in response to a gate signal. The luminance of the pixel is proportional to a magnitude of the driving current flowing in the OLED.
The organic light emitting display includes a data driving circuit to convert digital video data into analog data voltages and supplies the analog data voltages to data lines of a display panel. Because the data driving circuit generally has as many output channels as the data lines of the display panel, the size of the data driving circuit increases as the number of data lines increases. This results in a higher cost of the data driving circuit. In a related art device, a demultiplexer (demux) driving method using a demux switching circuit was proposed to reduce the number of output channels of the data driving circuit by an order of 2 or more.
FIG. 2 shows a related art 1 to 2 demux driving method. A demux switching circuit shown in FIG. 2 connects output channels CH1, CH2, and CH3 of a data driving circuit with data lines D1 to D6 of a display panel, connecting each output channel to two of the data lines through switches S11, S12, S21, S22, S31, and S32. The demux switching circuit time-divides a data voltage input through one output channel and supplies the data voltage to the two data lines. A time-division operation of the demux switching circuit is performed by switching operations of demux switches S11, S21, S31, S12, S22, and S32 driven in response to demux control signals DMUX1 and DMUX2. The first demux switches S11, S21, and S31 are simultaneously turned on in response to the first demux control signal DMUX1, and the second demux switches S12, S22, and S32 are simultaneously turned on in response to the second demux control signal DMUX2. In this instance, the first demux switches S11, S21, and S31 and the second demux switches S12, S22, and S32 are turned on at different times.
When the demux switch connected to the pixel changes from a turn-on state to a turn-off state, a parasitic capacitor may reduce a voltage applied to the pixel by a kickback voltage. Thus, first pixels connected to the first demux switches S11, S21, and S31 and second pixels connected to the second demux switches S12, S22, and S32 may be affected by the kickback voltage. In this instance, because the first demux switches S11, S21, and S31 and the second demux switches S12, S22, and S32 are turned on at different times, the number of times the kickback voltage influences the first pixels may be different from the number of times the kickback voltage influences the second pixels. This difference results in an unwanted current deviation between the first pixels and the second pixels. FIG. 3 shows a current deviation between adjacent pixels resulting from a difference between the numbers of times the kickback voltage influences the adjacent pixels. The current deviation generates a longitudinal dim and thus reduces image quality.
Each of pixels displaying red (R) includes a red OLED, each of pixels displaying green (G) includes a green OLED, and each of pixels displaying blue (B) includes a blue OLED. The R OLED, the G OLED, and the B OLED have different emission efficiencies. Thus, when the pixels generating the unwanted current deviation display different colors, the unwanted current deviation does not result in a longitudinal dim that is plainly visible. However, when the pixels generating the unwanted current deviation display the same color, the unwanted current deviation results in a longitudinal dim that is very noticeable. In other words, the problem of the current deviation is magnified when the pixels displaying the same color are selectively connected to the first demux switches S11, S21, and S31 and the second demux switches S12, S22, and S32 as shown in FIG. 2.